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User blog:Samuli.seppanen/Tale of amusing misfires
Past weekend I finished the final remaining tweaks to the cheiroballistra. Compared to the last session I had made a couple of changes: * Added a ~3 mm layer of wood on top of the slider (to ensure that bowstring does not strike the bolt too high) * Fixed angles of the field frames (one was tilted backwards) * Sank the claw into the slider * Made the contact between the claw and trigger tighter * Finished the new elliptically tillered arms with stronger hoops (increases strength of the cones significantly) So, assuming that everything would "just work" I shot the cheiroballistra a couple of times with the goal of improving previous session's 124 joule record. Instead I had a good round of "most amusing" misfires. The first shot broke my trusty ~48 gram test bolt to pieces: So it hit the target with the head pointing about 30 degrees left. So I adjusted the balance of the springs slightly to fix the problem, even though the center of the bowstring was no longer at the center. I went on and resumed tests with a lighter and shorter bolt (~28 grams). No help there. Bolt flight was erratic - not only did the bolt hit the target sideways, its butt-end seemed to have a tendency to tilt upwards. A couple of shots later I got by far the oddest misfire ever: It seems that the bowstring lifted the butt of the bolt up and moved it towards the left (as previously) to the degree that the head of the bolt actually burrowed into the slider before the bowstring flipped it up and ricocheted it towards the target from the roof. After this misfire it was time for some serious debugging. I noticed a couple things: * The hooks had started to unwind slightly. I assume this was because there was rather large (5mm) space between the cone and the curve in the field-frame, meaning that the bar had to endure a rather hefty blow at every shot. I worked around this by tying a piece of belt leather between the cone and the curve. * After each shot the bowstring had clearly risen ~8mm over the top of the slider. This seemed to explain why the butt-end of the bolt had a tendency to lift up. The reason seemed to be that bases of the cones were not vertically in the exact middle of the torsion bundle. I had always assumed that the cones auto-center on draw, but this does not seem to be the case. I guess the pressure of the springs is sufficient to keep them misaligned. So I forced the cones to the correct position and pumped the weapon a couple of times to ensure that the problem does no reoccur. It did not, fortunately. Now, with these fixes I felt confident that next shots would be perfect. So I (prematurely) setup the chronograph with its overpriced lights an all. And the result: Bolt was in pieces with multiple impact and "slide against metal" marks, one chronograph light support was badly bent and the power connector in the chronograph lights was broken. Fortunately I could fix the damage to the chronograph fairly easily and making a couple of new test bolts is fairly quick. After all this I was ready to point my finger at the torsion springs which, in the last session, had a slightly different linear pretension. As in: one of the springs had to be rotated a fair bit more to get the same power out of it. However, that difference seems to be gone. This must be related to the fact that one of the springs had been under tension for much longer than the other at the time of the previous session. Now both torsion springs have been sitting idle for months and difference in tension seems to have evened out. During pullback everything looked good before every single shot: the arms rotated the same amount and the center of the bowstring's center was in the correct place. All the components of the arms weight the same with 0,01 gram precision. So the only thing I can think of is that the tillering of the arms is slightly different, resulting in different weight placement. Which could result in one of the arms - most likely the right one - to rotate faster, and cause problems mentioned above. Fortunately this theory is easy to test by adding weight to the faster-moving arm, e.g. by wrapping some cord (or thin iron wire) near the hook for maximum effect and see if that helps. In all honesty I actually did manage to get one shot through the chronograph with the ~28 gram bolt. The energy was about 73 joules. While that seems disappointing, adding 1/3 to it gives ~100 joules. And I think that is doable, as it was a long-draw shot which have historically not given the best results. In fact, based on all the previous shooting sessions the arms should not be rotated much beyond ~110 degrees - or beyond ~90 degree angle to the case; the payback you get for the extra draw is much less than what you get by increasing washer rotation and using smaller draw. Unlike I originally thought this rule seems to apply even to a stomach-cocker like the cheiroballistra. As I have an article deadline coming up I really want to squeeze some good shots through the chronograph by end of the week. That would allow me to conclude the tests with nylon and say "it is now as good as it gets" and move on. Fortunately the cheiroballistra is still in perfect shooting condition, so there is hope. Category:Blog posts Category:Backup Category:Practical